Staphylococcus aureus produces a number of toxins of great medical importance, including toxic shock syndrome toxin (TSST-1) and enterotoxins B and C. Several of these toxins are carried on so-called pathogenicity islands, genetic elements that resemble prophages, but do not carry most of the genes normally associated with phage activities. These elements are normally highly stable, but specific phages have the ability to excise them and package them into phage particles that can transfer the toxin genes to new cells. SaPI1 is an S. aureus pathogenicity island that carries the genes for TSST-1 as well as enterotoxins Q and L. SaPI1 is mobilized by phage 80?, and efficiently packaged into phage-like particles, using 80? structural proteins. However, the SaPI1 capsid is only 1/3 the size of the normal 80? capsid, commensurate with its smaller genome. It is not known what the mechanism of size determination is, but the system resembles the E. coli phage P2/P4 system, in which the satellite phage P4 hijacks the structural proteins from P2 and uses them to package a smaller capsid. The overall objective of this study is to understand the mechanism of size determination in the S. aureus phage 80?/Sal1 system and its role in the development of bacterial pathogenesis. In this study we will use cryo-electron microscopy and three-dimensional reconstruction methods to determine structures of phage 80? and SaPI1 particles. The specific aims of this study are: (1) Determine the 3D structure of S. aureus phage 80? capsids; (2) Determine the 3D structure of SaPI1 capsids; (3) Determine the structure of procapsids produced by co-expression of 80?/SaPI1 structural proteins. RELEVANCE TO PUBLIC HEALTH: Staphylococcus aureus has become a major health problem in hospitals, especially with the emergence of multiple antibiotic resistant strains. Several of the toxins produced by pathogenic S. aureus cause severe systemic infections, including the toxic shock syndrome toxin TSST-1 and enterotoxins B and C. Recently, S. aureus toxins have become a particular concern as potential bioterrorism agents. The bacteriophage/pathogenicity island system is of special interest for its role in transferring toxin genes and conferring pathogenicity on non-pathogenic strains. [unreadable] [unreadable] [unreadable]